[NHN] and [OHO] hydrogen bonding in the adduct of 1,8-bis(dimethylamino)naphthalene (DMAN) with 1,8-dihydroxy-2,4-dinitronaphthalene (DHDNN)

X-Ray diffraction, IR and ¹H NMR studies were performed on the 1:1 adduct of 1,8-bis(dimethylamino)naphthalene (DMAN) with 1,8-dihydroxy-2,4-dinitronaphthalene (DHDNN). The adduct crystallizes in the triclinic system, space group , a = 9.911(2) Å, b = 11.212(2) Å, c = 11.194(2) Å, = 68.95(2)°, β = 79.72(2)°, γ = 73.78(2)°, Z = 2. Both [NHN]⁺ and [OHO]⁻ hydrogen bonds formed in the ion pairs are asymmetrical with lengths equal to 2.574(2) Å and 2.466(4) Å respectively. The [NHN]⁺ bridge shows a typical behaviour in the IR spectrum, i.e. a low-frequency absorption between 300 and 700 cm⁻¹. The coupling of [OHO]⁻ hydrogen bonds with the naphthalene π-electron system is so strong that no absorption related to the proton stretching vibrations can be detected in the high- and low-frequency regions. The ¹H NMR chemical shifts for the [NHN]⁺ and [OHO]⁻ bridge protons of 18.63 and 15.81 ppm respectively confirm the strong hydrogen bonds.     

NMR studies of solid state—solvent and H/D isotope effects on hydrogen bond geometries of 1:1 complexes of collidine with carboxylic acids

¹H and ¹⁵N NMR spectra of 10 complexes exhibiting strong OHN hydrogen bonds formed by ¹⁵N-labeled collidine and different proton donors, partially deuterated in mobile proton sites, have been observed by low-temperature NMR spectroscopy using a low-freezing CDF₃/CDF₂Cl mixture as polar aprotic solvent. The following proton donors have been used: HCl, formic acid, acetic acid, various substituted benzoic acids and HBF₄. The slow hydrogen bond exchange regime could be reached below 140 K, which allowed us to resolve ¹⁵N signal splittings due to H/D isotopic substitution. The valence bond order model is used to link the observed NMR parameters to hydrogen bond geometries. The results are compared to those obtained previously [Magn. Reson. Chem. 39 (2001) S18] for the same complexes in the organic solids. The increase of the dielectric constant from the organic solids to the solution (30 at 130 K) leads to a change of the hydrogen bond geometries along the geometric correlation line towards the zwitterionic structures, where the proton is partially transferred from oxygen to nitrogen. Whereas the changes of spectroscopic and, hence, geometric parameters are small for the systems which are already zwitterionic in the solid state, large changes are observed for molecular complexes which exhibit almost a full proton transfer from oxygen to nitrogen in the polar liquid solvent.     

Intramolecular hydrogen bonds and hydrogen-bonded systems in di-Schiff bases of 4-methyl-isophthalaldehyde with 4-substituted anilines

Three di-Schiff bases of 2-hydroxy-5-methyl-isophthalaldehyde with 4-R-anilines (R=H, CH₃, OCH₃) and their 1:1 complexes with HClO₄ were studied by FT-IR, ¹H, and ¹³C NMR spectroscopy in acetonitrile and [²H₃]acetonitrile solutions, respectively. In di-Schiff bases intramolecular OH…N hydrogen bonds have been detected; however, they show no proton polarizability. Hydrogen-bonded systems with fast proton fluctuation and large proton polarizability have been found in the 1:1 complexes of di-Schiff bases with HClO₄.     

Intramolecular hydrogen bonds in 3-diethylaminomethyl-5-R-salicylic aldehydes

Two 3-diethylaminomethyl-5-R-salicylic aldehydes were obtained and studied in chloroform solutions by FTIR and NMR spectroscopy. The existence of an equilibrium between the structures with OHO=C and NHO intramolecular hydrogen bonds was suggested. In the case of compound 1 (R=OCH₃) the OHO=C intramolecular hydrogen bond was more favorable whereas in the case of compound 2 (R=Br) the structure with the OHN intramolecular hydrogen bond was predominant.     

Crystal structure of Schiff base derivative of 2,2′-dihydroxybiphenyl-3-carbaldehyde with n-butylamine

Crystals of the Schiff base derivative of 2,2′-dihydroxybiphenyl-3-carbaldehyde with n-butylamine were examined using X-ray diffraction, FT-IR and CPMAS spectroscopy. Their space group is with a=8.377(2), b=12.214(2), c=14.774(3) Å, =76.62(3)°, β=81.34(3)°, γ=86.62(3)° and Z=4. The unit cell contains two symmetry-independent zwitterions. The hydrogen atom of the protonated N atom of the Schiff base is linked to the oxygen atom of the carbonyl group at position 2, which in turn is linked to the hydroxyl group by a short hydrogen bond [molecule A: NO=2.614(3), OO=2.520(3) Å; molecule B: NO=2.594(4), OO=2.526(3) Å]. The OHO⁻H⁺N bifurcated intramolecular hydrogen bonds are crystallographically asymmetric. The results of the FT-IR, ¹H, ¹³C, ¹⁵N NMR and CPMAS study of the crystals are in agreement with the X-ray data. Instead of a continuous absorption, only a broad band is found indicating relatively low proton polarizability in the two types of the cooperative relatively short intramolecular hydrogen bonds. The ¹⁵N NMR chemical shift indicates the protonation of the Schiff base.     

X-ray and spectroscopic studies of the molecular structure of bis(8-oxy-1-methylquinolinium) hydroiodide

Bis(8-oxy-1-methylquinolinium) hydroiodide has been studied by X-ray diffraction, FT-IR, ¹H and ¹³C NMR spectroscopy. In the crystalline state a homoconjugated OH  O⁻ hydrogen bond is formed. As indicated by the X-ray data this hydrogen bond is very short (2.457 Å) and structurally symmetrical. The existence of this bond is manifested in the FT-IR spectra by an intense and broad band in the 1500–400 cm⁻¹ region. This type of absorption indicates that this hydrogen bond can be described by one very broad potential energy minimum in which the proton shows a large proton polarizability due to its fast fluctuations, i.e., it shows the so-called Zundel’s polarizability.     

Contribution of the intramolecular hydrogen bond to the shift of the pKa value and the oxidation potential of phenols and phenolate anions

Intramolecularly OHO[double bond, length as m-dash]C hydrogen bonded phenols, 2-HO-C6H2-3,5-(t-Bu)2-CONH-t-Bu (1-OH), 2-HO-C6H2-5-t-Bu-1,3-(CONH-t-Bu)2 (2-OH) and 2-HO-C6H2-3,5-(t-Bu)2-NHCO-t-Bu (4-OH), were synthesized and their phenolate anions were prepared as tetraethylammonium salts (-1O-(NEt4+), 2-O-(NEt4+) and 4-O-(NEt4+)) with intramolecular NHO(oxyanion) hydrogen bonds. 4-HO-C(6)H(2)-3,5-t-Bu(2)-CONH-t-Bu (3-OH) and its phenolate anion, 3-O-(NEt4+), were synthesized as non-hydrogen bonded references. The presence of intramolecular hydrogen bonds was established through the crystallographic analysis and/or (1)H NMR spectroscopic results. Intramolecular NHO(phenol) hydrogen bonds shift the pK(a) of the phenol to a more acidic value. The results of cyclic voltammetry show that the intramolecular OH...O=C hydrogen bond negatively shifts the oxidation potential of the phenol. In contrast, the intramolecular NHO(oxyanion) hydrogen bond positively shifts the oxidation potential of the phenolate anion, preventing oxidation. These contributions of the hydrogen bond to the pKa value and the oxidation potentials probably play an important role in the formation of a tyrosyl radical in photosystem II.     

Structural role of hydrogen bond networks in amino acid–acid systems. (I) The network with highly polarizable OHO hydrogen bonds in sarcosine–methanesulfonic acid (2:1) crystal

The sarcosine–methanesulfonic acid (2:1) crystal was selected for examination of two problems: relations between different components of the amino acid–acid hydrogen bond network and a role of very strong and highly polarizable OHO hydrogen bond in the main structural units of the crystal: sarcosinium–sarcosine dimers (complexes). Our observations are based on phase transitions of the crystal monitored by DSC, X-ray diffraction and temperature evolutions of selected bands of IR spectra. Our experimental and DFT results provide information on the potential energy profile of the OHO proton and its evolution with temperature. The OO distance of the primary hydrogen bond remains almost unchanged and its proton is strongly delocalized and sensitive on neighbour NHO hydrogen bond. We propose a possible mechanism of the phase transitions and coupling between νCO vibrations of the carboxyl group and moving of the proton in neighbour OHO hydrogen bridge.     

Laser spectroscopic visualization of hydrogen bond motions in liquid water

Ultrafast pump-probe experiments are described permitting a visualization of molecular motions in diluted HDO/D₂O solutions. The experiments were realized in the mid-infrared spectral region with a time resolution of 150 fs. They were interpreted by a careful theoretical analysis, based on the correlation function approach of statistical mechanics. Combining experiment and theory, stretching motions of the OHO bonds as well as HDO rotations were ‘filmed’ in real time. It was found that molecular rotations are the principal agent of hydrogen bond breaking and making in water. Recent literatures covering the subject, including molecular dynamics simulations, are reviewed in detail.     

Formation of hydrogen-bonded chains between a strong base with guanidine-like character and phenols with various pKa values - an FT-IR study

The complexes formed by phenols with 1,3,4,6,7,8-hexahydro-l-methyl-2H-pyrimido[1,2-a]pyrimidine (mTBD), an N-base with guanidine-like character, were studied as a function of the pK_a of the phenols by FT-IR spectroscopy. The following phenols were used: 4-cyanophenol (4-CNPh), pentachlorophenol (PCP) and 2,6-dichloro-4-nitrophenol (DNPh). In the case of chloroform solutions of 1: 1 mixtures of the phenols with MTBD the corresponding complexes are formed completely. With increasing acidity of the phenols the hydrogen bonds become increasingly asymmetrical. The O … N ⁻O … H⁺N hydrogen bond in the 4-CNPh-MTBD complex shows large proton polarizability. In the other cases only the polar structure is realized. With increasing phenol MTBD ratio, the formation of chains with two phenol molecules is observed. With decreasing pK_a of the phenols the fluctuation is limited to the phenol-phenolate bond and finally, the phenol-protonated MTBD bond begins to dissociate. In acetonitrile solutions, N⁺H …O⁻ hydrogen bonds are observed in the case of the 1:1 mixture of 4-CNPh with MTBD. A weak continuum indicates the presence of homoconjugated phenol-phenolate bonds with large proton polarizability. In the case of 2:1 mixtures only protonated MTBD and homoconjugated phenol-phenolate bonds are observed, independent of the pK_a of the phenols. The results are discussed with regard to the proton pathway in bacteriorhodopsin.     

Formation of hydrogen-bonded chains between a strong base with guanidine-like character and phenols with various pKa values - an FT-IR study

The complexes formed by phenols with 1,3,4,6,7,8-hexahydro-1-methyl-2H-pyrimido[1,2-a]pyrimidine (mTBD), an N-base with guanidine-like character, were studied as a function of the pK_a of the phenols by FT-IR spectroscopy. The following phenols were used: 4-cyanophenol (4-CNPh), pentachlorophenol (PCP) and 2,6-dichloro-4-nitrophenol (DNPh). In the case of chloroform solutions of 1:1 mixtures of the phenols with MTBD the corresponding complexes are formed completely. With increasing acidity of the phenols the hydrogen bonds become increasingly asymmetrical. The OH … N ⁻O … H⁺N hydrogen bond in the 4-CNPh-MTBD complex shows large proton polarizability. In the other cases only the polar structure is realized. With increasing phenol MTBD ratio, the formation of chains with two phenol molecules is observed. With decreasing pK_a of the phenols the fluctuation is limited to the phenol-phenolate bond and finally, the phenol-protonated MTBD bond begins to dissociate. In acetonitrile solutions, N⁺H … O⁻ hydrogen bonds are observed in the case of the 1:1 mixture of 4-CNPh with MTBD. A weak continuum indicates the presence of homoconjugated phenol-phenolate bonds with large proton polarizability. In the case of 2:1 mixtures only protonated MTBD and homoconjugated phenol-phenolate bonds are observed, independent of the pK_a of the phenols. The results are discussed with regard to the proton pathway in bacteriorhodopsin.     

双碳醇水溶液的1H NMR实验与理论分析

In this study, ¹H nuclear magnetic resonance (NMR) measurements and quantum chemistry (QC) studies of ethanol (ET)-water mixtures and ethylene glycol (EG)-water mixtures are carried out at different temperatures to discuss the interactions between water and the alcohols present in the mixtures. From ¹H NMR spectra, it is observed that the chemical shift of the water proton shows two different trends in the ET-water mixtures and the EG-water mixtures. With increasing water concentration, the water proton chemical shift decreases dramatically for ET-water mixtures, while the chemical shift increases slowly for EG-water mixtures. The alcohol hydroxyl proton resonance peaks of both ET and EG shift to lower field with decreasing water concentration. It is found that the resonance peaks of all alkyl protons shift monotonically to low field with increasing alcohol concentration at different temperatures. The geometry optimization results indicate the formation of H-bonds between the water molecules and the hydroxyl groups of the alcohols alongside the weakening of O-H bonds in the alcohols, which results in an O-H bond length decrease. It is interesting to note that the bond length values computed for C-C, C-H and O-H bond in both ET and EG are larger when calculated at the density functional theory (DFT) (B3LYP) level than when calculated using Hartree-Fock (HF) level of theory with the same polarization function and diffusion function. However, the O-H…O H-bond computed at HF level of theory is stronger than that calculated at DFT level of theory. The theoretical results are in good agreement with the experimental ones. In the calculation of NMR chemical shift, DFT(B3LYP) is better than HF, which implies that for the same method, the larger the basis sets are, the more accurate are the calculated values.     

Shape of the proton potential in an intramolecular hydrogen-bonded system. Part II

The crystals of 5,5′-dibromo-3-diethylaminomethyl-2,2′-biphenol N-oxide were studied by X-ray and FT-IR spectroscopy. Within this molecule two short OHO intramolecular hydrogen bonds are formed. The NO?H⁺?O⁻ bond between the OH and the N-oxide groups is very strong, of 2.419(7) Å between the oxygen atoms. The proton potential of this hydrogen bond is flat, broad and has probably no barrier—consequently it could not be located from X-ray diffraction data. The other hydrogen bond formed between two hydroxyl groups appears asymmetrical from FT-IR spectra, and shows also relatively limited proton polarizability. The molecular conformation is non-planar, due to strong overcrowding effect between the oxygen atoms involved in the hydrogen bonds.     

Crystal structure of 1-hydroxy-1,2,3-benzotriazolium diphenyl phosphate reveals strong NHO and OHO hydrogen bonds

The structure of 1-hydroxy-1,2,3-benzotriazolium diphenyl phosphate (HOBt/DPP) has been investigated by X-ray analysis. The compound crystallizes in the monoclinic space group P2₁/n with Z = 4 and the following lattice parameters: a = 11.711(2), b = 12.727(2) and c = 12.794(3) Å, β = 105.12(2)°, V = 1840.9(6) ų. The structure was solved by direct methods and refined on F² to R values of wR2 = 0.084 and R1 = 0.034 for 1985 observed reflections. HOBt/DPP has an ionic structure with very short OHO and NHO hydrogen bonds linking the different ions. Owing to these hydrogen bonds, infinite screw-shaped chains which are twisted parallel to the y-axis are formed.     

Spectroscopic and PM5 semiempirical studies of new hydrazone of gossypol with 3,6-dioxaheptylhydrazine

A new hydrazone of gossypol with 3,6-dioxaheptylhydrazine (GHDO) has been synthesised and its structure has been studied by FT-IR, ¹H NMR, ¹³C NMR as well as PM5 semiempirical methods. All the studies have provided clear evidence of the existence of GHDO in the solution in the N-imine–N-imine tautomeric form. The structure and the spectroscopic behaviour of this tautomer are discussed in details. It is shown the structure of GHDO is strongly stabilised by different types of intramolecular hydrogen bonds. In two of them the oxygen atoms of the oxaalkyl chains are also engaged. The strongest intramolecular hydrogen bond is formed between the O₇H proton and N₁₆ atom from the hydrazone group.     

A study of the hydrogen bond by means of comparative calculations

The nature of the hydrogen bond is investigated by means of a comparative analysis of some hydrogen bonded and hydrogen-like bonded systems. It is concluded that the hydrogen bond is determined by electrostatic interaction between the proton and the region of high electron density in the neighbour molecule.     

Spectroscopic and semiempirical studies of new Schiff base of gossypol with allylamine in solution

The Schiff base of racemic gossypol with allylamine (GSBAL) has been studied by FT-IR, ¹H and ¹³C NMR spectroscopy as well as by the PM5 semiempirical method. The spectroscopic methods have shown that GSBAL Schiff base exists in chloroform solution as enamine–enamine tautomer. The structure of GSBAL and the hydrogen bonds within this structure have been calculated to show that the allyl groups are out-of-planes of naphthalene rings. The strongest intramolecular hydrogen bond within the structure of GSBAL is formed between O₇H–N₁₆ atoms and it belongs to the medium strong bonds. The other hydrogen bonds, although very weak, play a very important role in stabilising the GSBAL enamine–enamine structure.     

Coupling of functional hydrogen bonds in pyridoxal-5'-phosphate-enzyme model systems observed by solid-state NMR spectroscopy

We present a novel series of hydrogen-bonded, polycrystalline 1:1 complexes of Schiff base models of the cofactor pyridoxal-5'-phosphate (PLP) with carboxylic acids that mimic the cofactor in a variety of enzyme active sites. These systems contain an intramolecular OHN hydrogen bond characterized by a fast proton tautomerism as well as a strong intermolecular OHN hydrogen bond between the pyridine ring of the cofactor and the carboxylic acid. In particular, the aldenamine and aldimine Schiff bases N-(pyridoxylidene)tolylamine and N-(pyridoxylidene)methylamine, as well as their adducts, were synthesized and studied using 15N CP and 1H NMR techniques under static and/or MAS conditions. The geometries of the hydrogen bonds were obtained from X-ray structures, 1H and 15N chemical shift correlations, secondary H/D isotope effects on the 15N chemical shifts, or directly by measuring the dipolar 2H-15N couplings of static samples of the deuterated compounds. An interesting coupling of the two "functional" OHN hydrogen bonds was observed. When the Schiff base nitrogen atoms of the adducts carry an aliphatic substituent such as in the internal and external aldimines of PLP in the enzymatic environment, protonation of the ring nitrogen shifts the proton in the intramolecular OHN hydrogen bond from the oxygen to the Schiff base nitrogen. This effect, which increases the positive charge on the nitrogen atom, has been discussed as a prerequisite for cofactor activity. This coupled proton transfer does not occur if the Schiff base nitrogen atom carries an aromatic substituent.     

Weakly and strongly associated nonfreezable water bound in bones

Water bound in bone of rat tail vertebrae was investigated by ¹H NMR spectroscopy at 210–300 K and by the thermally stimulated depolarization current (TSDC) method at 190–265 K. The ¹H NMR spectra of water clusters were calculated by the GIAO method with the B3LYP/6-31G(d,p) basis set, and the solvent effects were analyzed by the HF/SM5.45/6-31G(d) method. The ¹H NMR spectra of water in bone tissue include two signals that can be assigned to typical water (chemical shift of proton resonance δ_H = 4–5 ppm) and unusual water (δ_H = 1.2–1.7 ppm). According to the quantum chemical calculations, the latter can be attributed to water molecules without the hydrogen bonds through the hydrogen atoms, e.g., interacting with hydrophobic environment. An increase in the amount of water in bone leads to an increase in the amount of typical water, which is characterized by higher associativity (i.e., a larger average number of hydrogen bonds per molecule) and fills larger pores, cavities and pockets in bone tissue.     

基于LFHB 理论模型关联和预测醇+惰性溶剂的1H NMR 化学位移

运用含氢键缔合的格子流体状态方程(LFHB), 仅用一个参数关联了一元醇-惰性溶剂共17个体系29套1H NMR化学位移数据. 并且用关联参数成功预测了不同温度下丁醇+环己烷的化学位移. 所得结果与化学缔合理论的结果进行了比较. 对于某些体系在稀浓度范围LFHB的计算氢键缔合度要低于化学缔合理论的结果. 并且分析了LFHB理论中的物理参数和化学参数对于缔合度计算的不同影响.